A Near Field Communication (NFC) or Radio Frequency Identification (RFID) communication system consists of two or more coupled resonant circuits. Those can be characterized by their resonance frequency, quality factors and coupling coefficients called system parameters.
The primary device (typically called “reader”) generates a magnetic RF field. The counterpart (which can be for example a mobile phone, a tag or a transponder) can be supplied with power via this RF field. The primary device modulates the RF field's amplitude for communication purposes. In order to communicate back to the primary device, the counterpart load modulates the primary device's RF field. This can be achieved either by passive or active load modulation.
The first is done by varying one or more system parameter according to the respective modulation scheme. This can be for example switching a resistor parallel to the counterpart's antenna (quality factor modulation). Passive load modulation is preferred for passive counterparts which are powered by the primary device's RF field.
In the case of active load modulation as described in US Patent Publication No. 20120071089 A1: “Method and Device for Active Load Modulation by Inductive Coupling”, the counterpart generates an RF field which is modulated according to the corresponding modulation scheme. Typically, the counterpart is a battery powered device. As a consequence, the modulation seen by the primary device can be by far stronger then by using passive load modulation.
System parameters and the selected method of load modulation affect the signal strength at the primary device's receiver input. Consequently, receiver units have to offer a large dynamic range in order to offer a stable communication performance for various counterparts featuring different types of antennas and modulation concepts.
NFC devices can act in initiator or target mode (e.g., ISO/IEC 18092 NFCP-IP1 or NFC Forum 2.0). In the initiator mode, the device generates an RF field (acts as a primary device”) whereas in the target mode, the device is a counterpart to a primary device. NFC devices typically share a common RF receiver frontend configuration for a communication technology. Hence, the dynamic range requirements are increased even more.